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Bureau of Mines Information Circular/1985 



Recognizing Signs of Excessive Loading 
in Underground Coal Mines 



By Eric R. Bauer 




UNITED STATES DEPARTMENT OF THE INTERIOR 



75 

A^;NES 75TH AV*^ 



Information Circular 9017 



Recognizing Signs of Excessive Loading 
in Underground Coal Mines 



By Eric R. Bauer 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Model, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 





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Library of Congress Cataloging in Publication Data: 



Bauer, Eric R 

Recognizing signs of excessive loading in underground coal mines. 

(Information circular / United States Department of the Interior, Bu- 
reau of Mines ; 9017) 

Bibliography: p. 29. 

Includes index. 

Supt. of Docs, no.: I 28.27:9017. 

1. Mine roof control. I. Title. II. Series: Information circular 
(United States. Bureau of Mines) ; 9017. 



TN295.U4 [TN2881 622s [622\8l 84-600355 



CONTENTS 

Page 

Abstract 1 

Introduction 1 

Acknowledgments 2 

Explanation of excessive loading 2 

Signs of excessive loading. 2 

Mine roof supports 2 

Timbers and posts .' 2 

Cribs 2 

Steel beams 2 

Wood beams 3 

Metal straps • 3 

Roof bolts 3 

Roof bolt plates 3 

Cap boards 3 

Cap wedges 16 

Header boards 16 

Yieldable steel legs 16 

Chocks and shields 18 

Mine installations 18 

Overcasts, brattices, and block structures 20 

Sealants 20 

Mine openings 20 

Roof 22 

Ribs 22 

Floor 22 

Conclusions 28 

References 29 

Appendix. — Glossary 30 

ILLUSTRATIONS 

1. Mine areas subject to excessive loading and associated signs of excessive 

loading 3 

2. Splitting wood timber 4 

3. Several bent (or buckling) wood timbers 5 

4 . Cracked wood timber 6 

5. Crushing wood crib, which also appears to be bending because of excessive 

vertical loading 7 

6 . Crushing and cracking concrete crib 8 

7. Steel beam bending at beam end 9 

8 . Steel beam twisting at the supported end 9 

9. Splitting or cracking wood beam 10 

10. Wood beam bending at its midspan 10 

1 1 . Crushing wood beam. 11 

12. Sheared metal strap. 11 

13. Bolt that has sheared or failed in tension because of excessive loading... 12 

14. Bending roof bolt plate 12 

15. Stress (strain) patterns on roof bolt plate as a result of excessive 

loading 13 

16. Roof bolt plate squeezing into a header board 13 

17. Splitting or cracking cap board 14 



11 



ILLUSTRATIONS— Continued 

Page 

18. Crushing cap board 15 

19. Split cap wedge and "brooming" of a timber 16 

20. Crushing cap wedges 17 

21. Cracked cap wedge 17 

22. Split or cracked header board 17 

23 . Bending header board 18 

24. Yield (crushing) of a yieldable steel leg 19 

25. Longwall support "going on the solid" (total collapse of the legs) because 

of excessive loading 19 

26. Longwall support punching into the roof and floor 19 

27. Cracking overcast 20 

28. Crushing block brattice 21 

29. Rib sealant that is cracking 22 

30. Rib sealant that is spalling 23 

3 1 . Roof being crushed by wooden support members 23 

32. Roof crack caused by excessive loading 24 

33. Shearing roof 24 

34. Roof sagging because of excessive loading 25 

35. Crushing rib 25 

36. Cracked rib 26 

37. Spalling rib 27 

38. Heaving and cracking mine floor 28 



RECOGNIZING SIGNS OF EXCESSIVE LOADING IN UNDERGROUND COAL MINES 

By Eric R. Bauer 



ABSTRACT 

This Bureau of Mines publication describes and illustrates the physi- 
cal signs of excessive loading on mine roof supports, installations, 
and openings. It is designed to assist mine personnel in recognizing 
potentially hazardous ground conditions, by emphasizing the preliminary 
effects of developing unstable ground. Increased worker awareness can 
be expected to reduce the injuries and fatalities resulting from falls 
of roof, rib, and face. 

INTRODUCTION 

Accidental falls of roof, rib, and face continue to be a leading 
cause of injuries and fatalities in underground mines. Statistics show 
that for the period of January 1978 through June 1984, underground coal 
mine fatalities due to ground control problems averaged 44.2 pet of all 
underground fatalities (1) ,^ This substantial percentage is a result 
of many factors but indicates that the ability of miners to identify 
developing hazardous ground conditions needs to improve before a reduc- 
tion in fatalities, injuries, and productivity loss can occur. 

The present publication examines the effects of excessive loading on 
mine roof supports, installations, and openings. Support items such as 
roof bolts and plates, steel beams, posts' and timbers, cribs, over- 
casts, and stoppings are examined. Conditions of roof, ribs and floor 
that suggest excessive loading are considered. Numerous photographs, 
taken in operating underground coal mines by Bureau of Mines personnel, 
supplement the text by providing actual examples of the conditions 
discussed. 

This publication is intended for mine operators and safety managers, 
and for use by training specialists in increasing the ability of exper- 
ienced miners to recognize hazardous roof conditions and in instructing 
new miners in the necessary techniques. 

^Mining engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 
^Underlined numbers in parentheses refer to items in the list of references preced- 
ing the appendix. 



ACKNOWLEDGMENTS 



The author wishes to thank the manage- 
ment and personnel of the following mines 
for their cooperation and permission to 
take photographs in their mines: Warwick 
No. 3 Mine, Greensboro, PA, Duquesne 



Light Co.; Kitt No. 1 Mine, Philippi, WV, 
Kitt Energy Corp.; Greenwich Colleries, 
Ebensburgh, PA, Pennsylvania Mines Corp.; 
Grove No. 1 Mine, Jennerstown, PA, G. M. 
and W. Coal Co. 



EXPLANATION OF EXCESSIVE LOADING 



Load (weight) on mine roof supports, 
installations, and openings occurs nor- 
mally as the seam is extracted, owing to 
the weight of the immediate roof , main 
roof , and overburden that acts on the re- 
maining support members. This loading 
normally occurs in the vertical plane 
but may be transferred to the horizontal 
plane. Ordinarily, the in-place supports 
(pillars, roof, and artificial supports) 
adequately support the load and no physi- 
cal signs of excessive loading appear. 



However, loading may exceed the support 
capabilities of the in-place supports.-^ 
Such excessive loading produces harmful 
effects that can be detected by visual 
observation. The miner must learn to 
recognize these effects as warning signs 
of potential roof falls, rib rolls, floor 
heave, etc. While excessive loading does 
not always lead to complete failure, it 
is a warning symptom that a serious 
hazard is developing and thus must be 
investigated. 



SIGNS OF EXCESSIVE LOADING 



The signs of excessive loading take 
many forms and involve numerous items and 
areas underground. These include mine 
roof supports (timbers and posts, cribs, 
wood and steel beams, cap boards and 
wedges, header boards, yleldable legs, 
and chocks or shields), mine installa- 
tions (overcasts, brattices, block struc- 
tures, and sealants), and mine open- 
ings (roof, ribs, and floor). The signs 
include splitting, bending, twisting, 
crushing, cracking, spalling, stressing, 
shearing, sagging, heaving, and squeez- 



ing. 



Figure 1 lists the areas affected 



and the associated signs of excessive 
loading, 

MINE ROOF SUPPORTS 

Mine roof supports are the physical re- 
inforcements installed to provide artifi- 
cial support of the exposed mine roof. 
Excessive loading can deform them and de- 
crease their supporting capability. De- 
formations and loss of support action may 

— 

-"For a more complete explanation of 
roof loading, support, and/or falls, see 
references 2-5. 



be gradual or sudden, depending on the 
type of support and the amount and rate 
of loading. In either case, visual signs 
of excessive loading usually appear prior 
to complete support failure. 

Timbers and Posts 

Excessive loading of wood timbers and 
posts can be recognized by splitting, 
bending (also known as buckling) , crack- 
ing, and punching into the roof and/or 
floor (figs. 2-4). 

Cribs 



Wood or concrete cribs reveal exces- 
sive loading by bending or crushing 
(figs. 5-6). 

Steel Beams 

Steel beams (I-beams or track rails) 
are placed in contact with the roof and 
held in position by wood posts or steel 
legs at the beam ends. Bending at the 
midspan and/or beam ends and twisting at 

^See appendix for definition of terms. 



Mine areas 

subject to 

excessive loading 


Signs of excessive loading 


llililiiilililfill 


Roof supports 


Timbers (posts) 


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Cribs 


:•; !•! 1 1 ; 1 i ; 


Steel beams 


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Wood beams 


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Metal straps 


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Roof bolts 


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Bolt plates 


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Cap boards 


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Cap wedges 


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Header boards 


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Yieldable legs 


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Chocks, shields 


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Installations 


Overcasts 


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Brattices 


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Block structures 


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Sealants 


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Openings 


Roof 


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Ribs 


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Floor 


i 1 i !•! 1 1 : l«l 



KEY 
# Areas affected and associated signs 
of excessive loading 

FIGURE 1. - Mine areas subject to excessive loading 
and associated signs of excessive loading. 

the beam ends are the signs of excessive 
loading (figs. 7-8). They may also indi- 
cate poor design practices , which accen- 
tuate the amount of loading. 

Wood Beams 



Wood beams provide support and are used 
in the same manner as steel beams. They 
show excessive loading by splitting or 
cracking, bending, and crushing (figs. 9- 
11). Defects in the beams may cause 
failure at low loads; thus, this must be 
considered prior to assuming that the 
failure indicates excessive loading. 



Metal Straps 

Metal straps, also known as bacon 
skins, steel headers, and steel mats, are 
bolted to the roof or rib and are de- 
signed to keep material from falling out 
around the bolts. They react to exces- 
sive loads by bending and shearing. The 
bending is difficult to detect because of 
the bends created during installation 
of the strap over uneven roof and ribs. 
Figure 12 shows an example of a sheared 
strap due to excessive loading. 

Roof Bolts 

Roof bolts are the primary method of 
artificial support employed in mines to- 
day and have a long history of successful 
use. Many problems can result from ex- 
cessive loading of roof bolts and roof 
trusses, most of which are unobservable 
and cannot be photographed. These in- 
clude anchor slippage, bending within the 
hole, stressing, loss of bonding, and 
torque loss or gain. The only detectable 
sign is the shearing or failure in ten- 
sion of the bolt or truss, most readily 
observable when it occurs at or near the 
head of the bolt or in the cross link of 
a truss (fig. 13). 

Roof Bolt Plates 



Roof bolt plates show excessive loading 
by bending, stressing, and squeezing. 
Bending is seen as the downward movement 
of the plate corners (fig. 14). Stress 
patterns, which develop as the load in- 
creases, can sometimes be seen in the 
rock dust or rust that accumulates on the 
plate (fig. 15). Squeezing is the in- 
denting of the plate into the roof or 
wood header board (fig. 16). 



Cap Boards 

Wood cap boards are 
the roof or floor and 
ports (timbers, posts, 
beams, etc.) to provide 
ing surface and better support. They in- 
dicate excessive loading by splitting 
or cracking and crushing (figs. 17-18). 



placed between 

artificial sup- 

, cribs, steel 

a larger bear- 




FIGURE 2. - Splitting wood timber. Timbers adjacent were installed at a later date. 
















FIGURE 3. - Several bent (or buckling) wood timbers. 



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FIGURE 4. - Cracked wood timber. 




FIGURE 5. - Crushing wood crib, which also appears to be bending because of excessive vertical 
loading. 




FIGURE 6. - Crushing and cracking concrete crib. The effects are due to the combination of exces- 
sive loading and differential settlement between the two sides. 




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FIGURE 7. - Steel beam bending at beam end. 




FIGURE 8. - Steel beam twisting at the supported end. This may also occur when a poorly designed 
beam, improperly installed, is subjected to small loads. 



10 







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FIGURE 9. - Splitting or cracking wood beam. 




FIGURE 10. - Wood beam bending at its midspan. Lateral loads, as well as vertical loads, may be 
the cause of this bending. 



11 





FIGURE 11. - Crushing wood bee 



m. 




FIGURE Vl. - Sheared metal strap. 



12 




FIGURE 13. - Bolt that has sheared or failed in tension because of excessive loading. 




FIGURE 14.- Bending roof bolt plate. Although this is an indication of excessive loading, the plate may 
be underdesigned for this application, resulting in a false indication of the loading present. 



13 




FIGURE 15. - Stress (strain) patterns on roof bolt plate as a result of excessive loading. 




FIGURE 16. - Roof bolt plate squeezing into a header board. 



14 





FIGURE 17. - Splitting or cracking cap board. Since tliis is a thick block thot would tend to 
fail at lower loads, this may not be a sign of excessive loading. 



15 




FIGURE 18. - Crushing cap board. Some crushing may be desirable to allow support-strata 
interaction; however, the amount shown is sufficient to indicate excessive loading. 



16 



Defects in the wood may lead to premature 
failure at lower loads, which must be 
considered before determining the loads 
responsible for the failure. 

Cap Wedges 



support. They will split or crack (fig. 
22), bend (fig. 23), and crush (fig. 16) 
when subjected to excessive loads. As 
before, the presence of defects must be 
considered before the final analysis is 
completed. 



Cap wedges show signs of excessive 
loading sooner than the supports with 
which they are used. This is because of 
the relative thinness of the wedges and 
the preloading (pressure) on the wedges 
at installation. Excessive loading is 
seen as splitting, crushing, and cracking 
(figs. 19-21), Again, defects as well as 
the material strength may be a factor in 
determining the loading that will cause 
failure. 

Header Boards 

Header boards are installed between the 
roof or rib and bolt plate to provide 
increased bearing surface and better 



Yieldable Steel Legs 

Yieldable steel legs are used as sup- 
port members for steel beams and in con- 
juction with arch assemblies. They yield 
at a sliding friction slip joint when 
their design friction limit is exceeded. 
Increased loading is indicated by either 
nonadjacent alignment marks (made on the 
legs at the time of installation) or by 
bare spots where the slip joint couplings 
were previously located. This increased 
loading is an indication of possible fu- 
ture excessive loads. Figure 24 shows 
the amount of yield (crushing) at a slip 
joint. 




FIGURE 19. - Split cap wedge and "brooming" of o timber. 



17 





FIGURE 20. - Crushing cap wedges. These are 
the initial indicators of future excessive loading. 



FIGURE 21. - Crocked cop wedge. This is on 
early indication of future excessive loading. 




FIGURE 22. - Split or cracked header board. 



18 




FIGURE 23. - Bending header board. 



The legs may also twist or bend, usu- 
ally after they have yielded as far as 
their design limit allows. By this time, 
roof conditions would have deteriorated 
so much that travel in the affected area 
would be prohibited; thus, no photographs 
of this condition are included. 

Chocks and Shields 

Chocks and shields provide roof support 
during longwall mining. They show exces- 
sive loading by crushing (also known as 
collapsing). This involves either "going 
on the solid" (total collapse of legs) 
(fig. 25) or punching into a soft floor 
or roof (fig. 26). 



MINE INSTALLATIONS 

A second area where evidence of exces- 
sive loading is found is mine installa- 
tions, such as overcasts, brattices, 
block structures, and sealants. Exces- 
sive loading appears primarily as crush- 
ing or cracking; sealants are prone to 
spalling as a result of excessive loading 
on mine pillars. 

As in the case of mine roof supports, 
the effects on mine installations develop 
either slowly or rapidly, depending on 
the type of installation and the amount 
and rate of loading. The effects are 
usually observable prior to complete 
structural failure of the installation. 



19 





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FIGURE 24. - Yield (crushing) of a yieldable steel 
leg. 



B 

FIGURE 25. - Longwall support "going on the sol- 
id (total col lapse of the legs) because of excessive 
loading. A, Front leg collapse because of roof frac- 
ture at longwall face; B, rear legcollapse because of 
failure of roof to cave properly. 




FIGURE 26. - Longwall support punching into the roof and floor. 



20 



Overcasts, Brattices, 
and Block Structures 

Overcasts are used to Isolate intake 
air from return air. Block brattices 
(stoppings, bulkheads, seals, etc.) are 
placed in crosscuts to separate air cur- 
rents and force the air ahead until it 
reaches the desired points (6, pp. 212- 
213). They are also used to isolate 
abandoned workings from active workings. 
Block structures include pump houses, 
transformer stations, retaining walls, 
battery charging stations, underground 
shops, and other cement block structures. 

Overcasts, brattices, and block struc- 
tures show excessive loading by crushing 
or cracking of the cement block and mor- 
tar from which they are constructed. An 
example of a cracking overcast is shown 
in figure 27, and a crushing brattice is 
shown in figure 28. 



Sealants 

Mine sealants are applied to mine roof, 
ribs, and block structures to minimize 
the effect of the wet-dry cycle of ven- 
tilating air and to strengthen the area 
sprayed. They reveal excessive loads 
by cracking and spalling (figs. 29-30). 
Cracking and spalling may also be a re- 
sult of improper application of the seal- 
ant, which would cause premature failure 
at lower loads. 

MINE OPENINGS 

The final area that must be examined 
for signs of excessive loading are the 
mine openings. These include the roof, 
rib, and floor. They can crush, crack, 
shear, sag, spall, or heave when loaded 
beyond their ultimate strength. De- 
terioration of mine openings is the 





FIGURE 27. - Cracking overcast. 



21 




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FIGURE 28. • Crushing block brattice. 



22 



"beginning of the end," since it is usu- 
ally irreversible and will progress until 
access to the area becomes unsafe or 
impossible. 

Roof 

Mine roof can show excessive loading in 
several ways. The most detectable signs 
are crushing, cracking, shearing, and 
sagging (figs. 31-34). Of these, sagging 
is the most difficult to detect without 
using sophisticated instrumentation. 



Ribs 

For mine ribs , signs of excessive load- 
ing are crushing, cracking, and spalling 
(figs. 35-37). 

Floor 

Mine floor shows excessive loading by 
cracking and heaving, which usually occur 
simultaneously (fig. 38). 




FIGURE 29. - Rib sealant that is cracking. 



23 




FIGURE 30. - Rib sealant that is spalling. 




FIGURE 31. - Roof being crushed by wooden support members. 



24 




FIGURE 32. - Roof crock caused by excessive loading. 




FIGURE 33. - Shearing roof. 



25 



Roof bolts 




Not to scale 



FIGURE 34. - Roof sagging because of excessive loading. 




FIGURE 35. - Crushing rib. 



26 




FIGURE 36. - Cracked rib. 



27 




FIGURE 37. - Spoiling rib. 



28 



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FIGURE 38. - Heaving and cracking mine floor. 



CONCLUSIONS 



Many safety areas must be addressed by 
government and industry in the effort to 
reduce injuries and fatalities due to 
roof, rib, and face falls, A most impor- 
tant area is the training of miners to 
routinely look for and recognize the 
signs of hazardous ground conditions, in- 
cluding excessive loading on supports, 
installations, and openings in under- 
ground mines. This publication is de- 
signed for use by mine safety instructors 
engaged in such training and as a refer- 
ence guide for mine personnel. 

It must also be noted that what appears 
on the surface to be excessive loading 
may be the result of defects in the sup- 
port members. Load measuring devices are 



available with which to supplement the 
visual observations, and their use may be 
necessary before a final determination of 
the cause and amount of loading can be 
made. 

Finally, roof, ribs, and support mem- 
bers can fail suddenly without revealing 
any evidence of overloading. Although 
this is the exception rather than the 
rule, an area must not be assumed to be 
completely safe just because no visual 
signs of excessive loading are present. 
All miners must be continuously aware of 
their surroundings and any changes that 
occur, no matter how small the changes 
may be. 



REFERENCES 



29 



1. Mine Safety and Health Administra- 
tion (Dep. Labor). Mine Injuries and 
Worktlme, Quarterly. Closeout Edition, 
1978-1982, p. 17; Closeout Edition, 1983, 
p. 18; Jan. -June, 1984, p, 17. 

2. Forbes, J. J., E. Thomas, and A. J. 
Barry. Questions and Answers on Roof 
Support in Bituminous-Coal Mines. U.S. 
Dep. Interior Handbook, 1951, 90 pp. 



5. Caudle, R. D. Mine Roof Stability. 
Paper in Ground Control Aspects of Coal 
Mine Design. Proceedings: Bureau of 
Mines Technology Transfer Seminar, Lex- 
ington, Ky. , March 6, 1973. BuMines IC 
8630, 1974, pp. 79-85. 

6. Cassidy, S. M. (ed.). Elements of 
Practical Coal Mining. Soc. Min. Eng. 
AIME, 1973, 614 pp. 



3. Mine Enforcement and Safety Admini- 
stration (now Mine Safety and Health Ad- 
ministration (Dep. Labor)). Programmed 
Instruction Workbook No. 3, 1976, 111 pp. 

4. Morgan, T. A. Coal Mine Roof Prob- 
lems . Paper in Ground Control Aspects of 
Coal Mine Design. Proceedings: Bureau 
of Mines Technology Transfer Seminar, 
Lexington, Ky. , March 6, 1973. BuMines 
IC 8630, 1974, pp. 56-61. 



7. Woolf, H. B. (ed.). Webster's New 
Collegiate Dictionary. G. & C. Merriam 
Co., 1979, 1532 pp. 

8. Thrush, P. W. A Dictionary of Min- 
ing, Mineral, and Related Terms. BuMines 
Spec. Publ., 1968, 1269 pp. 



30 



APPENDIX. —GLOSSARY 1 



Bending — The turning or forcing of a 
support member from a straight shape 
to a curved. Sometimes referred to as 
buckling. 

Cracking — The breaking, splitting, or 
snapping apart of a support member such 
that a fissure (crack) appears. 

Crushing — The squeezing or forcing by 
pressure of a support member so as to al- 
ter it into a mass or destroy it. The 
failure of a material by compression. 

Heaving — The upward lifting of the mine 
floor. The rising of the bottom after 
removal of the coal. 

Sagging — The drooping, sinking, set- 
tling, or bowing of the mine roof, usu- 
ally maximum at the center of the span. 
Curving downward in the middle, usually 
as a result of improper loading or 
supporting. 

Shearing — The cracking of the mine roof 
because of stress resulting from applied 
forces that cause two contiguous roof 
parts to slide relative to each other. 

^Definitions are taken or adapted from 
references 7 and 8, which appear in the 
list preceding the appendix. 



The separation of a roof bolt into two 
distinct pieces because of stress. The 
cutting of a metal strap by a roof bolt 
or roof bolt plate. 

Spalling — The breaking off of chips , 
scales, or slabs of mine ribs, sealants, 
or roof. Flaking off of rock, mineral, 
or metal from its surface. 

Splitting — The lengthwise dividing of a 
mine support, usually along the grain or 
seam. 

Squeezing — The forcing or thrusting by 
compression (imbedding) of a roof bolt 
plate into a header board or mine roof 
because of the exertion of pressure on 
opposite sides. The effect of the clo- 
sure of an entry on supports. Sometimes 
referred to as punching. 

Stressing — The formation of stress pat- 
terns on roof bolt plates because of the 
force exerted when the plate presses 
on, pulls on, or pushes against another 
structure. The resistance of a body to 
compressional, tensional, or torsional 
force. 

Twisting — The turning or shape changing 
of a support member under torsion and as- 
suming of a partially spiral shape. 



t^U.S. CPO: 1985-505-019/20,038 



INT.-BU.OF MINES, PGH., PA. 27951 



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